When I was editing an illustrator file, I got quite a scare when all of sudden “undo” doesn’t work anymore. Not just the cmd+z doesn’t work, the whole “undo” function is ineffective. After taking some deep breath, when I was able to think again, I noticed that “undo” was only ineffective after I changed certain objects, in this case, a symbol. Changes of other elements were fine. I noticed a little framed “+” icon at the right bottom corner of the symbol thumbnail, now I learn that it is an icon to indicate that it is a “dynamic symbol”. Something like this:
Changing the symbol to “Static Symbol” fixed all the problems!!
Now I learned that “What you can’t do is move, delete, or even scale those individual elements.”
Learn more about dynamic symbol: Using Illustrator’s New Dynamic Symbols
I hope it helps you if you encountered something similar.